The demands on automobiles for better fuel economy have been increasing in recent years as concern with environmental issues has been rising. Good fuel economy is also being required of the rubber compositions used for automotive tires. For example, rubber compositions containing a conjugated diene polymer (e.g., polybutadiene, butadiene-styrene copolymer) and a filler (e.g., carbon black, silica) are used for the rubber compositions for automotive tires.
Patent Literature 1, for example, proposes a method for enhancing the fuel economy. This method uses a diene rubber that has been modified with an organosilicon compound containing an amino group and an alkoxy group. These days, however, further enhancement of the fuel economy has been demanded. Meanwhile, wet-grip performance and abrasion resistance are also properties required of the rubber compositions for automotive tires; however, these properties generally assume an inverse relationship with the fuel economy. Thus, it has been difficult to achieve the respective properties at high levels in a balanced manner.
Moreover, in order to meet the demand for fuel economy, silica-containing rubber compositions have been used in not only treads but also other various components. However, silica has a lower affinity for rubber (in particular, natural rubber, butadiene rubber, styrene butadiene rubber, and others that are often used for tires) than carbon black due to hydrophilic silanol groups on its surface. Thus, silica is often inferior in terms of abrasion resistance and mechanical strength (tensile strength, elongation at break).
Use of a silane coupling agent and use of highly reinforcing silica such as finely divided silica are known as the methods for improving the above properties.
However, such highly reinforcing silica generally exhibits low dispersion in rubber compositions. Therefore, the abrasion resistance and mechanical strength may not be much improved, or even may be deteriorated.
Conventional silane coupling agents generally used in rubber compositions for tires (e.g. bis(3-triethoxysilylpropyl)disulfide, bis(3-triethoxysilylpropyl)tetrasulfide) greatly improve the dispersibility of silica to impart good mechanical properties. However, since a large amount of silane coupling agent is necessary to favorably disperse highly reinforcing silica, the cost increases greatly. Moreover, the addition of even a sufficient amount of silane coupling agent may fail to provide good dispersion.
Use of a mercapto group-containing silane coupling agent has been proposed which has higher reactivity than the aforementioned conventional coupling agents (for example, see Patent Literature 2). Although such highly reactive silane coupling agents have high performance, they cause the scorch time to be considerably shortened, making their practical use difficult in the tire industry. Therefore, currently such silane coupling agents are hardly ever used.
Patent Literature 3 discloses a silica-containing rubber composition for a tire that can enhance the wet-grip performance without deteriorating the performance in terms of rolling resistance and abrasion resistance. However, this rubber composition still needs to be improved in terms of enhancing these properties in a balanced manner.